1. Field of the Invention
This invention relates to a lined storage tank equipped with a leak detection and monitoring system, and more particularly a leak detection and monitoring system for use with ionic solutions stored in mobile storage tanks.
2. Prior Art
Over the road tanks, such as tanks carried by tractor trailers and rail cars, are often utilized to carry ionic solutions from one place to another. Some of the solutions are hazardous, and could dissolve through the steel or aluminum tanks if exposed to them. Accordingly, a liner is utilized to line the inside of the tanks to prevent exposure of the transported solution with the metal making up the tank.
Over time, and especially if solutions are carried which are incompatible with the lining in the tank, the lining may wear out. When the solution contacts the metal tank, it may start a chemical reaction to rapidly corrode a hole through the tank which could result in a spill of hazardous material. Avoiding this problem is a concern for over-the-road transportation companies.
Presently, there are two primary ways which are utilized to test over-the-road tanks for leaks when full of liquid. First, a conductivity test may be conducted. A Milliamp (mA) meter is connected to a battery (a direct current source), such as a five volt battery. The meter may be a Simpson analog meter, or any other suitable device. The meter is put into test mode and adjusted until it reads 3 mA. The meter is then connected to a probe which is placed in a liquid and the other connection is grounded to the rail car, or shell of a tank. The test button is then pushed and if the reading is higher than 3 mA, a leak is present, but if it is lower than 3 mA, no leak is reported.
The conductivity test is not a particularly precise test and it provides little advance warning before having a relatively large problem. Additionally, the test is performed by taking an access cover off the top of the tank, and dropping an electrode into the transported solution. Many items inadvertently end up in a tank including watches, wrenches, bolts, etc. . . . over time when the tanks have an exposed opening. Furthermore, since the system operates on direct current, the possibility of polarization exists, as the resistant increases, the current decreases. Polarization of the probe occurs through use which would provide indications that the liner is good, while actually defective. If the operator were aware of the polarization, the leads could be reversed, but a need exists for a monitoring system which does not necessarily rely on the skill or experience of the employee to operate properly. A large spill could subject the transportation company to large liabilities.
The second way commonly utilized to test tank liners is to perform a megohm (megger) check where a large voltage is applied across a first electrode placed in the solution in the tank, and another electrode is placed in contact with the tank (opposite the liner) from the first electrode. The current passing through the electrodes is measured and a resistance value is provided for the xe2x80x9ccircuitxe2x80x9d, i.e., through the liner. If the resistance drops below a certain value, such as 10,000 ohms, then a leak is present. If above, the cutoff, then no leak is present.
The problem with the megger test is that the resistance measured is not direct resistance but an equivalent resistance through all parallel circuits. Accordingly, if a pin hole leak were present offering a minimal amount of resistance, such as 0.1 ohms and the remainder of the liner provided excellent resistance such as 10,000 ohms, then in a large tank, the equivalent resistance may be on the order of 9,000 ohms, which would not be reported as a leak.
The most accurate way presently utilized to check for leaks in an empty tank is to perform a spark test. A probe having 15,000 volts is passed across the liner. If a leak is found, even a pin hole leak, a visible spark travels from the probe through the hole to the tank. The problem with this method is that most liners are applied in sheets like wallpaper and overlap adjacent sheets. The spark will not travel very far between two sheets, while a leak may travel a few feet through the adjacent liner layers to the tank wall. Government Regulation No. 4 M 183 requires a certified inspector to perform this check and DOT requirements require the test to be performed yearly on tractor trailer tanks. There are no known regulations addressing rail car tests.
Accordingly, a need exists for a leak detection system for use with tanks, especially mobile tanks.
It is an object of the present invention to provide a lined tank equipped with a monitor able to detect very small leaks in a liner.
It is another object of the present invention to provide a method and apparatus for monitoring and recording data relating to the performance of a tank liner.
Another object of the present invention is to provide an alarm to an operator of a vehicle in the event of a breach of a liner in a mobile tank.
Another object of the present invention is to provide a regular monitoring of a lined tank to check for a breach in the integrity of a liner.
Another object of the present invention is to utilize the natural potential difference between dissimilar metals to provide a voltage in the event of a breach of a liner.
Another object of the present invention is to provide a method and apparatus for monitoring a mobile lined tank for breaches in the liner when the tank is filled with an electrically conductive liquid.
Accordingly, a tank, preferably on a wheeled vehicle such as a railcar or trailer, is provided with a protective liner. Two electrodes are preferably permanently mounted within the tank and respectively connected to a monitor. Another two ground connections are made from the monitor to the tank. At least one of the two probes in the tank are utilized with the monitor to measure the voltage between the tank and the selected probe. When the tank is filled with an ionic solution, a breach of integrity of the liner results in a voltage corresponding to the difference in potentials of the metals forming the probe and tank is read by the monitor. This triggers an alarm. Furthermore, the monitor provides a voltage from probe to probe, probe to shell, and shell to shell on a periodic basis to measure and then record the performance of the liner. The effective or equivalent resistance of the liner will be recorded in a memory of the monitor which may be downloaded to a computer for monitoring the performance of the liner. Any breaches of the liner whether obtained from the galvanic cell measurement or equivalent resistance measurement are provided to an alarm.